notebook: Latest preact-compat release breaks notebook.

Three of our javascript dependencies, preact, preact-compat, and proptypes, are currently managed via unpkg links, and pinned with ^ rather than ~. ^ appears to have the same semantics as in npm, in which it means “allow non-major version upgrades”.

The most recent release of preact-compat appears to have broken the notebook. If I clear my ~/.cache/bower and then run python setup.py css js in an editably-installed notebook repo, when I run jupyter notebook I get a blank page on the notebook page, with console errors about missing createContext from preact-compat:

I think the easiest fix here is to pin via ~, which seems to be the convention followed for most other js dependencies (MathJax seems to be a notable exception to this rule). We may also want to report this upstream as a bug to preact-compat? I don’t really have a good sense of whether that’s desirable.